The 3GPP, which is an organization that standardizes third generation mobile phone systems, has been making a study generically called LTE (Long Term Evolution), to achieve dramatic improvement in transmission speed and reduction in transmission delay in a radio access network (RAN). The 3GPP has been developing standard specifications for constituent techniques for such study.
As shown in FIG. 5, a radio access network (E-UTRAN: Evolved Universal Terrestrial RAN) in a mobile communication system employing the LTE scheme includes a mobile station UE (User Equipment) and a radio base station eNB (E-UTRAN Node B). The mobile station UE and the radio base station eNB communicate with each other through a radio link (RL).
Each of the mobile station UE and the radio base station eNB terminates an RLC (Radio Link Control) sublayer, a MAC (Medium Access Control) sublayer, and a physical (PHY) layer.
A transmitting side apparatus (the mobile station UE or the radio base station eNB) subjects transmission data to RLC processing, MAC processing, and PHY processing, in this order, and then transmits the data as a radio signal from a radio unit.
On the other hand, a receiving side apparatus (the mobile station UE or the radio base station eNB) extracts transmission data by subjecting the radio signal received at a radio unit to PHY processing, MAC processing, and RLC processing, in this order.
Here, the transmission data includes user data (U-plane data) generated by applications or the like used by the user; and control data (C-plane data), such as RRC (Radio Resource Control) signaling and NAS (Non Access Stratum) signaling, used for control of the mobile communication system.
Moreover, RLC retransmission control processing is performed between the RLC sublayer of the transmitting side apparatus and the RLC sublayer of the receiving side apparatus; and HARQ (Hybrid Automatic Repeat Request) retransmission control processing (MAC retransmission control processing) is performed between the MAC sublayer of the transmitting side apparatus and the MAC sublayer of the receiving side apparatus.
In a mobile communication system employing an IMT-2000 scheme, as shown in FIG. 1, the MAC sublayer of a receiving side apparatus performs reordering processing on RLC-PDUs received, and sends the RLC-PDUs to the RLC sublayer in order of the sequence numbers.
In contrast, in a mobile communication system employing the LTE scheme, as shown in FIG. 2, the MAC sublayer of a receiving side apparatus sends received RLC-PDUs to the RLC sublayer without performing the reordering processing on the RLC-PDUs.
Accordingly, in the mobile communication system employing the LTE scheme, as FIG. 3 shows, the RLC sublayer of the receiving side apparatus is configured to perform reordering processing on received RLC-PDUs as follows. Specifically, when receiving an RLC-PDU of sequence number #4 before receiving an RLC-PDU of sequence number #3, the RLC sublayer activates a reordering timer and waits for receipt of the RLC-PDU of sequence number #3 until the reordering timer expires.